Cobalt-Chromium Alloy (Stellite 6)

Use abrasive cut-off wheel with adequate coolant flow. Standard cut-off wheel (1.0-1.5 mm thickness) is appropriate. Use adequate coolant flow to prevent overheating. Cutting speed: 200-300 RPM for most cut-off saws. Apply steady, moderate pressure. Avoid forcing the cut which can cause wheel wear and sample damage. Leave adequate allowance (~2-3 mm) for grinding away the heat-affected zone from cutting.

Cold mounting with epoxy resin is preferred to avoid heat that could affect the microstructure. Use a low-shrinkage epoxy resin for best edge retention. Ensure complete cure before grinding to prevent edge rounding and maintain sample integrity.

Hot compression mounting is acceptable if the part tolerates ~150-180°C and moderate pressure (2000-3000 psi for phenolic). Use phenolic or epoxy-phenolic resins. Ensure proper cooling under pressure to minimize shrinkage.

The hardness (450 HB) of Cobalt-Chromium Alloy (Stellite 6) requires careful grinding. Use standard SiC grinding papers with adequate water lubrication. Disc speed: 200-300 RPM. Apply light to moderate pressure (30-40 N per 30 mm sample) - the hard material may require longer grinding times. Use sharp, fresh grinding papers to minimize deformation.

Grinding sequence:

• 120 grit: Remove sectioning damage (40-90 seconds). Use moderate pressure to remove heat-affected zone.
• 240 grit: Remove previous scratches (40-90 seconds). Ensure complete scratch removal.
• 320 grit: Remove previous scratches (40-90 seconds). Ensure complete scratch removal.
• 400 grit: Remove previous scratches (40-90 seconds). Ensure complete scratch removal.
• 600 grit: Remove previous scratches (40-90 seconds). Ensure complete scratch removal.

Always rotate the specimen holder 90° between steps to ensure complete scratch removal. Use complementary rotation (platen and holder same direction, different speeds) rather than contra-rotation to minimize deformation. Adequate water lubrication is critical - avoid drying during grinding which can cause smearing.

The hardness requires careful polishing. Use diamond polishing with appropriate polishing pads for each stage. Apply light to moderate pressure throughout to prevent deformation.

Diamond polishing sequence:

• 6μm diamond: 2-4 minutes on a medium-hard synthetic pad (e.g., TEXPAN) with light to moderate pressure (30-40 N per 30 mm sample). Start with 6μm to minimize damage. The hard material may require longer polishing times.
• 1μm diamond: 2-4 minutes on a medium-hard synthetic pad (e.g., TEXPAN) with light pressure (25-35 N). Continue removing scratches from previous step.
• 0.05μm colloidal silica: 1-2 minutes on a high-napped final polishing pad (e.g., MICROPAD) with very light pressure. This removes any remaining fine scratches and prepares the surface for etching. Monitor for relief - reduce polishing time if excessive relief develops.

Use appropriate polishing lubricants. The hard material means polishing times should be sufficient but not excessive - avoid over-polishing which can cause relief and affect grain boundary revelation. Monitor the surface frequently under the microscope to check for smearing or excessive relief.

Cobalt-Chromium Alloy (Stellite 6) responds well to standard etchants for cobalt alloy materials. The cobalt-chromium matrix with carbides will reveal clearly with appropriate etchants. Glyceregia (Chemical Etching) - Primary choice for general microstructure:

• Composition: 10ml glycerol, 15ml HCl, 5ml HNO₃
• Preparation: Add acids to glycerol slowly with stirring. Prepare fresh for best results. The glycerol moderates the reaction rate.
• Application: Immerse sample or swab for 10-30 seconds. Classic general-purpose micro-etchant for austenitic stainless steels and nickel alloys.
• Reveals: Grain boundaries, grain structure, and twin boundaries clearly. Excellent for general microstructure examination.
• Rinse: Immediately with water, then ethanol. Dry with compressed air or warm air to avoid staining.
• Note: Prepare fresh when needed. Shelf life: several hours. Use in fume hood.

Electrolytic 10% Oxalic (Chemical Etching) - Standard etchant for this material:

• Application: Follow standard procedures for Electrolytic 10% Oxalic.
• Reveals: Grain boundaries and microstructure clearly.
• Rinse: Immediately with water, then ethanol. Dry with compressed air.
• Note: Use appropriate safety measures. Consult material-specific guidelines.

Etching Strategy:

• Start with Glyceregia for general microstructure examination
• Always clean and degrease before etching
• Use short initial etch times (a few seconds), check under the microscope, repeat if needed
• Check etching progress frequently - over-etching can obscure fine details

Safety: All etchants require proper PPE (gloves, safety glasses, lab coat), proper fume hood, and appropriate safety measures. Handle with care.